This invention relates to a process for the preparation of thermodynamically stable metallic contacts for binary oxide-, nitride-, carbide- or phosphide-semiconductors. The product of this invention, the metal-semiconductor contact, is a crucial building block in the making of any electronic device based on these semiconductors that is intended to be operated at high temperatures. In particular, this method is useful for making metal contacts to group III nitrides and to silicon carbides.
Metal contacts to modem silicon devices typically incorporate a transition-metal/silicide contacting layer. The formation of transition-metal silicides and their characteristics have been exhaustively investigated, and their current manufacturing technology provides a satisfactory solution for contacting silicon devices. On the other hand, for compound semiconductors, the search for contacting solutions is still widely pursued, while the primary obstacle is the added complexity of ternary reactions.
Despite the rapid built-up of ternary phase diagram knowledge base during the last decade, the ability to use it for stable contact design is limited. For example, the applicability of phase diagrams is limited to closed thermodynamic systems.
Maintaining a closed system is difficult in compounds such as Group III arsenides and nitrides since one of the elements is volatile. The complexity is also manifested in the reaction kinetics. As a result, in systems, such as silicon carbides, the obtained contact is typically composed of a mixture of phases, which is of limited use in the sub-micron regime and can affect the electrical characteristics of the contact.
An extensive technology of semiconductor devices has been developed based upon the properties of silicon and other materials with comparable band gap which may be doped, heat treated, and otherwise processed to produce adjacent layers and regions of varying electronic characteristics.
The use of devices produced from such semiconductors is generally limited to operation at ambient or, at most, moderately elevated temperatures and in non-corrosive, inert atmospheres. The temperature limitation is a consequence of the small band gap of these materials,(1-1.5 eV) which leads to large leakage currents and device failure at elevated temperatures. In addition, rapid diffusion of dopants and/or impurity species in the host material can occur, which in turn can substantially alter the character of the fabricated semiconductor device.
The limitation to relatively inert environments results from the high chemical reactivity of moderate band gap semiconductors (including silicon) to many corrosive environments, which also can alter the character of the fabricated device.
Such devices are also limited as to power level, frequency, and radiation tolerance by the materials used therein. For some applications, the temperature, environmental, and other use limitations on such devices may be overcome by the use of proper cooling and packaging techniques.
In other applications, these limitations have prevented the use of silicon for integrated circuit technology. For example, in many spacecraft and aircraft applications, elevated temperatures are encountered, and it is not always possible to insure that adequate cooling will be provided. In high power applications, device temperatures can rise to levels, which degrade or destroy the device solely through internal heating.
Silicon""s inability to withstand high temperatures limits the amount of power, which can be generated or controlled by silicon electronics. In addition, internal thermal transients in devices otherwise operating at ambient temperature can rapidly destroy the operability of the device unless extensive cooling is provided. Such cooling requires that the device be larger in size than might otherwise be necessary, in part defeating the purpose of the integrated circuit technology.
There has therefore been an ongoing, but as yet not fully successful, search over a period of twenty years to identify and develop a semiconductor technology based in other materials. Such a technology would desirably allow the fabrication of devices for use at higher temperatures such as, for example, the range of at least about 400xc2x0 C. to 600xc2x0 C., and in applications not amenable to the use of silicon. Because corrosive effects can be greatly accelerated at elevated temperatures and pressures, any such materials and devices must also exhibit excellent corrosion resistance at the elevated use temperatures and over a range of pressures from vacuum to many atmospheres.
Some generally desirable characteristics of such materials have been identified, including large band gap., good electrical conductivity, high electric field breakdown strength, low dielectric constant, ability to be doped to produce regions of varying electronic characteristics, a high melting temperature, good strength at operating temperatures, resistance to diffusion by undesired foreign atoms, good thermal conductivity, thermal stability, chemical inertness, and the ability to form stable ohmic and rectifying external contacts.
The metal-semiconductor contact is a crucial building block in the making of any electronic device based on these semiconductors that is intended to be operated at high temperatures. However, at elevated temperatures the metal semiconductor junction appears to become unstable.
Attempts to achieve limited stability have been made using complex contacts composed of several layers, one of which is usually used as a diffusion barrier. A typical such contact is the Ti Pt Au contact, where the Pt layer serves as a diffusion barrier to prevent the reaction of Ti and Au. This and similar contacts have been used on most of the aforementioned binary semiconductors. However, this contact has a very limited lifetime at high temperatures ( greater than 300xc2x0 C.) at which diffusion eventually occurs and the undesirable reaction takes place.
Silicon carbide serves as an example for a material meeting the indicated requirements of a semiconductor for high temperatures. Silicon carbide has a high decomposition temperature, good strength., good resistance to radiation damage and good corrosion resistance in many environments. Silicon carbide has a high breakdown field strength ten times that of silicon, a relatively large band gap, low dielectric constant, and a thermal conductivity of more than three times that of silicon at ambient temperature. The diffusion coefficients in silicon carbide are also much smaller than those in silicon or gallium arsenide, and so silicon carbide is resistant to the diffusion of impurity species. Silicon carbide may be processed by several techniques similar to those used in silicon device technology and in many instances silicon carbide devices may be substituted at moderate and low temperatures for silicon devices. Silicon carbide semiconductor device technology therefore offers the opportunity for supplementing, and in some instances replacing, conventional silicon device technology.
U.S. Pat. No. 5,442,200 to Tischler discloses a method to avoid the formation of more than one phase in contacting SiC using a sacrificial layer of Si.
A near-noble metal reacts with Si at temperatures lower than needed to start their reaction with SiC. This difference in temperatures can be used to obtain a full reaction with the Si while avoiding the reaction with the SiC. Thus, a silicide contact that is stable with the SiC substrate may be formed without the second phase of the carbon product that is formed in silicidation of SiC.
However, this type of contact may only serve as an ohmic contact, since it does not provide an xe2x80x9cin-situxe2x80x9d formation of a new interface such as in the case of Si silicidation, which is required for high quality and reproducible Schottky barriers. Furthermore, at the high temperatures needed to obtain the ohmic characteristics described in U.S. Pat. No. 5,442,200 (1100xc2x0 C.), a reaction with the SiC substrate may take place nevertheless. This reaction is probably quite limited since the inventor states that the final metallization may contain up to 5% carbon.
Levit et al. in J Appl. Phys. 80, 167 (1996), show that with a high temperature annealing the 10% Ti component of a Ni-Ti alloy contact react preferentially with the carbon product formed upon SiC silicidation with the Ni component of the alloy. Tile low percentage of Ti was deliberately made small to form a small perturbation to the Nixe2x80x94SiC system.
However, this contact does not provide a solution for the carbon product due to the very small quantity of Ti. Moreover, the preparation of the alloy is a complicated process as compared to the more common layer-by-layer deposition.
It is therefore the object of the invention to provide a thermodynamically stable electrical contact to wide band gap binary semiconductors.
It is another object of the invention to provide such stable contacts having either ohmic or rectifying properties.
It is yet another object of the invention to provide a thermodynamically stable electrical contact to semiconductor devices operating at elevated temperatures.
It has now been found that a bi-layer metallic contact, composed of sequentially deposited layers of a reactive refractory transition metal element, i.e., a metal selected from column 4b in the periodic; table, such as Ti, Zr or Hf, followed by a near-noble metal, i.e., a metal selected from group 8 in the periodic table, such as Pt, Pd, Co, or Ni, can be used to form the desired thermodynamically stable contacts to oxide-, nitride-, phosphide- or carbide-binary compound semiconductor.
Upon annealing in vacuum at high temperatures (typically 900xc2x0 C.), a reaction takes place between the semiconductor and the bi-layer metallization. The refractory metal reacts to form a compound with the less metallic element of the substrate, while the near-noble metal bonds with the other element, forming a distinct layer for each phase in a thermodynamically stable sequence.
In the cases where the less metallic element is volatile, the formed compound between the refractory metal and the less metallic element serves as a diffusion barrier to block further out-diffusion and decomposition of the substrate. In the cases where the less metallic element is non-volatile, this layer serves as a sink for the undesired product: that otherwise would have provided a second phase at the contact.
According to the invention there is provided a solid-state electronic device, comprising: (a) a binary substrate including a first element A and a second element D, D being less metallic than A, and, (b) an electrical contact including: (i) a first layer, in contact with the substrate, the first layer including a first binary compound of A with a metallic element X, and (ii) a second layer, in contact with the first layer, the second layer including a binary compound of D with a metallic element Z.
According to the invention there is provided an electrical contact for a binary substrate that includes a first element A and a second element D, D being less metallic than A, comprising: (a) a first layer, in contact with the substrate, the first layer including a first binary compound of A with a metallic element X; and (b) a second layer, in contact with the first layer, the second layer including a binary compound of D with a metallic element Z.
According to the invention there is provided a method for preparing a contact for a binary substrate including a first element A and a second element D, D being less metallic then A, comprising the steps of: (a) depositing a layer of Z metal on the substrate; (b) depositing of a layer of metal X on top of the layer of metal Z and, annealing the substrate with the deposited layers of metal X and of metal Z at a sufficient high temperature for a sufficient length of time to allow A and D of the substrate to react with the metal X and with the metal Z to yield the binaries of A with X and of Z with D respectively.
According to the invention there is provided a method for preparing a contact for a binary substrate that includes a first element A and a second element D, D being less metallic then A, comprising the steps of: (a) depositing a layer including a metal Z and a metal X on the substrate; and (b) annealing the substrate with the deposited layer at a sufficiently high temperature for a sufficient length of time, to allow A and D of the substrate to react with the metal X and with the metal Z to yield a first layer, in contact with the substrate, of a binary compound of A and X, and a second layer, in contact with the first layer, of a binary compound of D and Z.